A process known as upmixing involves deriving some number M of audio signal channels from a smaller number N of audio signal channels. Some audio processing devices capable of upmixing (which may be referred to herein as “upmixers”) may, for example, be able to output 3, 5, 7, 9 or more audio channels based on 2 input audio channels. Some upmixers may be able to analyze the phase and amplitude of two input signal channels to determine how the sound field they represent is intended to convey directional impressions to a listener. One example of such an upmixing device is the Dolby® Pro Logic® II decoder described in Gundry, “A New Active Matrix Decoder for Surround Sound” (19th AES Conference, May 2001).
The input audio signals may include diffuse and/or directional audio data. With regard to the directional audio data, an upmixer should be capable of generating output signals for multiple channels to provide the listener with the sensation of one or more aural components having apparent locations and/or directions. Some audio signals, such as those corresponding to gunshots, may be very directional. Diffuse audio signals, such as those corresponding to wind, rain, ambient noise, etc., may have little or no apparent directionality. When processing audio data that also includes diffuse audio signals, the listener should be provided with the perception of an enveloping diffuse sound field corresponding to the diffuse audio signals.